The present invention relates to analysis of electrocardiogram signals and, more particularly, but not exclusively to a method and apparatus for enhancement of electrocardiogram signals, with clinical applications.
According to the world health organization (WHO) the leading cause of mortality and morbidity worldwide is heart disease. This fatal disease is a big challenge to the health system and the main goal is the detection of the disease at a young age (at the 4th-5th decades) when the mortality rate is higher. At the other end an early detection and diagnosis of the disease can lead to an effective treatment, which may lead to reduction in both mortality and morbidity for many people worldwide.
The Electrocardiogram as a Heart Diagnosis Tool
As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body. This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG, or EKG). A typical ECG tracing for a single heartbeat is illustrated in FIG. 1. A waveform envelope 10 having several different components represents the heartbeat. The different components or waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles.
The P wave 12 represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.1 seconds (80-100 ms) in duration. The brief isoelectric (zero voltage) period 14 after the P wave represents the time in which the impulse is traveling within the AV node where the conduction velocity is greatly retarded.
The period of time 16 from the onset of the P wave to the beginning of the QRS complex is termed the P-R interval, which normally ranges from 0.12 to 0.20 seconds in duration. This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. If the P-R interval is >0.2 sec, a conduction defect (usually within the AV node) is present, a condition known as first-degree heart block.
The QRS complex 18 represents ventricular depolarization. The duration of the QRS complex is normally 0.06 to 0.1 seconds. This relatively short duration indicates that ventricular depolarization normally occurs very rapidly. If the QRS complex is prolonged (>0.1 sec), conduction is impaired within the ventricles. This can occur with bundle branch blocks or whenever a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. Such an ectopic foci nearly always results in impulses being conducted over slower pathways within the heart, thereby increasing the time for depolarization and the duration of the QRS complex. The QRS complex comprises a short negative Q wave 18.1, a large positive R wave 18.2 and a short negative S wave 18.3.
The isoelectric period (ST segment) 20 following the QRS is the time at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential. The ST segment is important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.
The T wave 22 represents ventricular repolarization and is longer in duration than depolarization, meaning that conduction of the repolarization wave is slower than the wave of depolarization.
The Q-T interval 24 represents the time for both ventricular depolarization and repolarization to occur, and therefore roughly estimates the duration of an average ventricular action potential. This interval can range from 0.2 to 0.4 seconds depending upon heart rate. At high heart rates, ventricular action potentials shorten in duration, which decreases the Q-T interval. Because prolonged Q-T intervals can be diagnostic for susceptibility to certain types of tachyarrhythmia, it is important to determine if a given Q-T interval is excessively long. In practice, the Q-T interval is expressed as a “corrected Q-T (Q-Tc)” by taking the Q-T interval and dividing it by the square root of the R-R interval (interval between ventricular depolarizations). This allows an assessment of the Q-T interval that is independent of heart rate. Normal corrected Q-Tc intervals are less than 0.44 seconds.
There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization. Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger ventricular-generated QRS complex.
ECG tracings recorded simultaneous from different electrodes placed on the body produce different characteristic waveforms. The routine twelve leads electrocardiogram is an old and well-established technique and is very useful in every-day clinical activity. This method can show an active event like myocardial infarction (“heart attack”) at its very acute stage or other cardiac ischemic events at time of occurrence (like in the so called “unstable coronary syndrome”). Routine ECG can also show that the heart underwent infarction in the past. This method cannot detect the existence of narrowing of the coronary arteries (“coronary disease”) in a quiescence situation and can appear totally normal even though there is a sever underling disease. To detect the coronary disease prior to damage to the heart there are several provocative tests that bring the heart to a maximal workload while testing its response to the stress.
There are several routine “non-invasive” methods to detect and diagnose heart disease based on provocative tests. One method is electrocardiogram monitoring during programmed exercise aimed to achieve target high heart rate. Several electrocardiogram changes at the peak heart rate (“ST segment deviation”) may indicate heart ischemia usually caused by narrowing of the coronary arteries supplying blood to the heart muscle. This method is not highly sensitive and it can detect the coronary disease in only 50-60%.
Another technique couples the programmed ECG monitored exercise with the injection of radio-nuclear compound (usually Thallium 207) at the time of peak heart rate. Reduction in blood supply to a certain heart segment diminishes the Thallium concentration in this segment. The radioactivity from the heart is detected by Gamma camera and a cold spot represent a segment with reduced perfusion due to narrowing in the coronary artery supplying this segment. The sensitivity of this method to detect coronary disease is 80-90% but it is an expensive method requiring high-price equipment that demands special training and a prolonged learning curve and is complicated by the need to use and handle radioactive compounds.
Another diagnostic method is the stress echocardiogram. This technique utilizes echocardiogram imaging of the heart during peak heart contraction achieved by either exercise or infusion of the drug dobutamine. Under this peak heart workload a reduction in contractility of a particular segment is an indication of reduction in blood supply to this segment due to coronary artery narrowing. This method has a sensitivity of 70-80%. Still the data acquisition requires expensive equipment, cumbersome detection and reading of the heart contraction and is associated with prolonged training and learning curve.
Recently there have been preliminary reports of the use of non-invasive multi-slice computerized tomography (CT) angiograms using X rays to detect coronary disease. This method can show the absence of coronary disease but the method has a low predictive value in detecting the disease and its severity. This is an expensive device and the patient is exposed to high amounts of contrast media and X ray radiation.
In the routine regular rest ECG in use the doctor is shown a series of ECG signals showing numerous heartbeats in succession obtained from 12 leads. Each lead records the heart activity from a different angle and shows 3 to 7 heartbeats in succession. The rest ECG using few heartbeats in each lead has limited diagnostic power. It can show damage to the heart in the past or the occurrence of an acute event such as evolving myocardial infarction (“heart attack”) or rhythm disturbances. Rest ECG can be diagnosed as normal but the patient may have a severe coronary disease underlying. Thus the rest ECG has no prognostic power and it cannot detect coronary disease as long as the heart itself is not damaged. However by obtaining many heartbeats at each lead using this embodiment the ECG is enhanced and several new features emerge with better diagnostic and prognostic power.
There is thus a widely recognized need for, and it would be highly advantageous to have, an electrocardiogram monitoring and analysis system devoid of the above limitations.